Nanoparticle dyes boost storage

By
Eric Smalley,
Technology Research NewsShrinking the size of fluorescent dye particles
is one way to cram more information into optical data storage devices,
but dye particles can be made only so small.

Researchers from the University of Toronto have found a way to
put layers of different dyes into a single particle, making it possible
to record several bits of data on the same spot. The technique could increase
data storage capacities several-fold, and improve anticounterfeiting measures.

The researchers' method involves a thin film of particles that
can be applied to a surface. Data is recorded on the film by shining lasers
tuned to specific wavelengths, or colors, at particular particles to activate
the fluorescent dyes they contain. The activated dyes glow when exposed
to lower-intensity light tuned to a second wavelength. Different dyes
are activated and read by different wavelengths of light.

The microscopic dye particles could be used to print large amounts
of information onto surfaces of documents, photographs and packaging,
said Eugenia Kumacheva, an associate professor of chemistry at the University
of Toronto. Detailed codes, patterns or images can be used as unique,
difficult-to-duplicate, identifiers. And dyes that can be read only under
ultraviolet or infrared light can be used to make the recordings invisible.

The particles can also be used to cram a large amount of information
onto optical data storage media, according to Kumacheva. "These materials
are equally important for high-density optical data storage and encryption
of different patterns for personal identification for security purposes,"
she said.

The key to cramming more information in the same space is making
the particles in layers, with each layer containing a different dye. The
researchers made particles containing two and three dyes. Each dye is
mixed with a different type of polymer. Two-dye particles contain a 700-nanometer
diameter core and a 150-nanometer thick shell. Three-dye particles require
an additional 50-nanometer-thick inner shell. A nanometer is one millionth
of a millimeter. In comparison, a red blood cell is 5,000 nanometers in
diameter.

Using two-dye particles, a given spot can hold two bits of data
rather than one, and a three-dye particle film is capable of recording
three bits per spot. "This approach leads [to] a much higher density in
data storage [that] would be difficult to counterfeit," said Kumacheva.

The researchers' scheme avoids a problem common in similar work,
said Kumacheva. Even with different light sensitivities, an unilluminated
dye particle can gain energy from a nearby illuminated particle. "Other
light-sensitive materials including dyes or pigments have [not] allowed
multiple writing on the same spot without crosstalk between the patterns,"
she said.

Because the multi-dye core-shell nanoparticles are fluorescent
at several specific wavelengths, they could also be used to make tunable
lasers and other photonic devices, according to Kumacheva.

The researchers are working on optimizing the recording procedure
and finding better combinations of dyes, said Kumacheva. The printing
method could be used for practical applications in five years, she said.

Kumacheva's research colleagues were Hung H. Pham, Ilya Gourevich,
Jung Kwon Oh and James E. N. Jonkman. The researchers presented the work
at the Particles 2003 conference in Toronto in August. The research was
funded by The Canada Research Chair Fund (NSERC Canada).